Yes it is well worth the trip. Book online and collect your ticket(s) by presenting your card in the machine in the tickets office.
The online booking//ticket will give you a time for your ride turn up 20 minutes previous to this grab your ticket and jump the main Q. No wasted time.
Westminster is the best Tube station as it is a short walk over the Thames by Parliament. From there buy a street map and walk Whitehall, The Mall Trafalgar Square Buck House for the best expereinces.